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Re: [OT] fork() 50 times slower on some Linux kernels?

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On 09-Jan-12 18:22, Kaz Kylheku wrote:
> On 2012-01-09, Jens Thoms Toerring <jt@toerring.de> wrote:
>> Kaz, sorry for the "[OT]" but I prefer it that way;-)
>> Stephen Sprunk <stephen@sprunk.org> wrote:
>>>> In any case, did you notice in the original posting that this 1 second
>>>> pause occurs on a system with 8GB of RAM?
>>>
>>> That is apparently how long it takes for the kernel on his system to
>>> find backing for all those pages.  I suspect that delay is why
>>> overcommitment is on by default.
>>
>> I now see what you meant in you're earlier post, but - as a
>> completely unscientific experiment - switching off overcom-
>> mitment doesn't seem to make any difference on my system.
> 
> Finding backing for pages happens during the memset operation, whereas
> what is being timed is the call to system().
> 
> Even with strict accounting, fork should not be duplicating the pages and
> committing memory for them, but be implementing a copy-on-write optimization.
> Strict accounting would mean that fork should fail if creating the (empty!)
> maps for the process would bring the total VM footprint of the system over the
> limit.
> 
> That's the theory.

If you don't back those pages during the fork() operation, then you're
back to the original problem.

For instance, imagine a process that allocates a thousand pages.  If
overcommitment is disabled, then the kernel will actually back them (or
allocation will fail).  The process forks, and then both parent and
child write to each of the thousand pages, triggering a thousand
copy-on-writes and the need to find a thousand backing pages.  If even
one of those pages can't be backed, then one of the processes will
fail--something that should have been impossible.  (Or worse, the OOM
Killer turns its attention elsewhere to obtain backing pages...)

A simpler implementation would be to reject copy-on-write entirely,
duplicating the pages.  However, that would unnecessarily double the
number of active pages and pollute the cache even if no writes occur,
which is the normal case.

S

-- 
Stephen Sprunk         "God does not play dice."  --Albert Einstein
CCIE #3723         "God is an inveterate gambler, and He throws the
K5SSS        dice at every possible opportunity." --Stephen Hawking

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fork() 50 times slower on some Linux kernels? Elmar <elmar.krieger@gmail.com> - 2012-01-09 12:25 -0800
  Re: fork() 50 times slower on some Linux kernels? Ian Collins <ian-news@hotmail.com> - 2012-01-10 09:41 +1300
  Re: fork() 50 times slower on some Linux kernels? Kaz Kylheku <kaz@kylheku.com> - 2012-01-09 21:32 +0000
    [OT] Re: fork() 50 times slower on some Linux kernels? jt@toerring.de (Jens Thoms Toerring) - 2012-01-09 22:12 +0000
      Re: [OT] Re: fork() 50 times slower on some Linux kernels? Kaz Kylheku <kaz@kylheku.com> - 2012-01-09 22:32 +0000
        Re: [OT] Re: fork() 50 times slower on some Linux kernels? Ian Collins <ian-news@hotmail.com> - 2012-01-10 11:56 +1300
  Re: fork() 50 times slower on some Linux kernels? Stephen Sprunk <stephen@sprunk.org> - 2012-01-09 16:11 -0600
    Re: fork() 50 times slower on some Linux kernels? Kaz Kylheku <kaz@kylheku.com> - 2012-01-09 22:30 +0000
      Re: fork() 50 times slower on some Linux kernels? Stephen Sprunk <stephen@sprunk.org> - 2012-01-09 17:20 -0600
        Re: [OT] fork() 50 times slower on some Linux kernels? jt@toerring.de (Jens Thoms Toerring) - 2012-01-09 23:57 +0000
          Re: [OT] fork() 50 times slower on some Linux kernels? Kaz Kylheku <kaz@kylheku.com> - 2012-01-10 00:22 +0000
            Re: [OT] fork() 50 times slower on some Linux kernels? Stephen Sprunk <stephen@sprunk.org> - 2012-01-09 20:39 -0600
              Re: [OT] fork() 50 times slower on some Linux kernels? William Ahern <william@wilbur.25thandClement.com> - 2012-01-09 19:43 -0800
                Re: [OT] fork() 50 times slower on some Linux kernels? Stephen Sprunk <stephen@sprunk.org> - 2012-01-10 00:31 -0600
          Re: fork() 50 times slower on some Linux kernels? Elmar <elmar.krieger@gmail.com> - 2012-01-11 14:39 -0800
        Re: fork() 50 times slower on some Linux kernels? Nobody <nobody@nowhere.com> - 2012-01-10 23:07 +0000

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